Proteins derived from the mos, myb, ras, and sis oncogenes have been expressed in Escherichia coli by use of the expression plasmid, pJL6, and its derivatives. This plasmid incorporates the strong, well-regulated, phage Lambda pL promoter and a small amino-terminal fragment of the Lambda cII gene. For each of the proteins made, the size is close to that predicted from the DNA sequence. Thus, the E. coli expression system provides an independent confirmation of the presence and length of open reading frames. The c-sis protein was derived from cDNA cloned from human T-cell leukemia virus-transformed cells (HTLV). The expressed bacterial protein was shown to be the expected c-sis product as judged by its reactivity with antibodies raised against purified sis peptides. The Lambda sequences in pJL6 were placed adjacent to the E. coli Beta-galactosidase gene (lacZ) by in vitro recombination, such that the lacZ sequence is out of frame with respect to the lambda cII sequence. A restriction enzyme recognition site (NruI) is located between cII and lacZ, allowing the insertion of DNA fragments. Inserted open reading frames that are properly aligned with the cII and lacZ sequences can be easily identified, since plasmids containing such sequences confer a lac+ phenotype on the host bacterium.